Let’s start learning about trainer kites for Kiteboarding, first you need to know: what is a trainer kite?
Trainer kites are smaller versions of larger kiteboarding kites and are a fantastic way for beginners to get started. A trainer kite is designed to teach you how to master the control of a kite, without the power of a full-sized kiteboarding kite. There are 3 main types of trainer kites on the market: with 2 lines*, 3 lines and the 4 lines kite

2 Lines trainer kite
This type of kite is a ram-air (foil) kite, with 2 lines connected to a set of bridles. We consider this type of kite to have fallen out of usage due to the surge of better trainer kites.
3 Lines trainer kite
With this trainer kite the third line is connected to the center of the trailing edge, slides through the bar and then is connected to a safety leash, usually attached to the wrist. Instead of watching your kite fly away if you let go of the bar, the kite will flag out and fall on the ground.
While some kites have obvious limitations, a 3.5 meter trainer kite (and sometimes smaller) could get you moving on a land board, a snowboard, and even some kite buggies.
4 lines trainer kites
Their leading edge is usually inflatable (LEI). They have 2 steering lines attached to the wing tips (back lines) and 2 power lines attached to bridles which are themselves attached to the leading edge (front lines). You need a harness and a safety leash to fly those kites, as one of the front line is the safety line, attached to the safety leash. They are basically full-on kites, in a size that you would require more than 40 knots of wind for an average adult to be able to ride. You can find them from 2 to 6 square meters.
Note: The 2- or 3-lines trainer kites used to be a mandatory step in kiteboarding schools, but now the majority of instructors have replaced them with a small 4 lines kite on short lines, because the experience is much closer to the reality of flying a kiteboarding kite.
My personal advice is to have a clear idea of what you hope to get out of the experience of Kiteboarding; are you more interested in casually learning or do you have your sights set on a more extreme high-flying experience?
If you are a really enthusiastic learner and you want to spend a long time working on kite control until fully mastering the winds, then yes you should buy a Trainer Kite, I recommend you to buy a small 4 lines kite with short lines or ask your IKO certified instructor who will know exactly the kite you’ll need based on your skills and experience and also will teach you how to exercise with your own Trainer Kite without catching bad habits with the kite.